“SURPRISE!!! WE ARE PREGNANT!!! This was the hardest secret we’ve ever had to keep,” Kerr, 28, captioned her post. “This journey for us has been full of many happy times & also many tears… The biggest rollercoaster we have ever been on, without a doubt! All of the moments of heartache were followed by the biggest blessing in the entire world!! We will never take one moment for granted & can’t wait to meet you, sweet baby!!!”

Added Aldean, “Been hard to keep this secret but we couldn’t be happier to add to our family. This year just gets better and better. #bunintheoven”

The new baby will join Aldean’s two daughters from his previous relationship with Jessica Ussery, whomhe split from in April 2013 after 12 years of marriage: Kendyl, 9, and Keeley, 14.

The musician and mom-to-be began dating in March 2014, tying the knot a year later in Mexico. For the beachfront nuptials, the bride wore a long-sleeved lace gown and carried a bouquet of yellow, red and purple flowers.

“My heart has never been more full of happiness and love,” Kerr posted on Instagram following the couple’s September 2014 engagement announcement. “Can’t wait to spend the rest of my life with this man. So many blessings and memories are in our future and SO thankful I get to share them with him.”

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Aldean spoke to PEOPLE in October about dealing with negative comments surrounding his relationship, saying, “People are very opinionated about things they’re not really aware of.”

“But at the same time I’m not going to spend all my time defending myself. I am what I am,” added the “Burnin’ It Down” singer. “People that know me or know my situation, they actually know.”

One positive ingredient? The message behind Aldean’s latest album They Don’t Know, which highlights the happiness he is feeling in his life.

“On this album, instead of focusing on all that other stuff that was going on for so long, everything is good and it made a world of difference for me,” he said of the project. “Anytime you’re in a place where you can relax and when you’re happy, that’s going to make everything else better.”

“I know she would like to have at least one kid,” Aldean added in October of his wife. “And I’m down for that. I love kids and being a dad is one of the best things in the world for me. If it’s meant to happen, it will. Only time will tell.”

“Keeley’s always been a mother hen type with babies, so we’d have a built-in babysitter!” he shared. “And Kendyl has always been the little sister, so for her to be the big sister, I think she’d be pretty stoked.”